Automatic cutting condition changeover equipment

ABSTRACT

An automatic cutting condition changeover equipment for automatically changing over an electrical cutting condition of a wire-cut, electric discharge machine in accordance with the thickness of a work. The equipment is provided with a feed control circuit for controlling the cutting speed of the work so that a mean working voltage between a wire electrode and the work may be constant, an arithmetic logic unit for calculating a predetermined optimum cutting condition formula on an on-line basis, and electrical cutting condition changeover control circuit for changing a peak current of the power source of the wire-cut, electric discharge machine so that the output from the arithmetic logic unit may be reduced to substantially zero.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to automatic cutting condition changeoverequipment which automatically changes the electrical cutting conditionof a wire-cut, electric discharge machine in accordance with thethickness of the workpiece.

2. Description of the Prior Art

A wire-cut, electric discharge machine cannot achieve optimum cutting ofthe workpiece unless an electrical cutting conditions of the machine,which depend on the peak current Ip or the no-load voltage Vs of thepower source or on the ON time τ_(on) and OFF time τ_(off) of a pulsecurrent is changed sharply in accordance with the thickness of theworkpiece. For example, in the case of the peak current Ip being notvaried in accordance with the work piece thickness if a workpiece ofsmall thickness is cut under the same conditions as those for aworkpiece of large thickness, then a wire electrode will be broken;conversely, if the thick workpiece is cut under the same conditions asthose for the thin workpiece, then a sufficient cutting speed will notbe obtained. But manually changing the electrically cutting conditionsfor each particular workpiece is very troublesome to the operator and,further, complicates continuous, fully automatic cutting of a workpieceof uneven thickness. To solve such problems, there has already beenproposed and put to practical use automatic cutting conditionchange-over equipment which is capable of automatically changing overthe electrical cutting condition in response to variations in thethickness of a workpiece during cutting.

According to the conventional automatic cutting condition changeoverequipment, for instance, cutting speeds corresponding to selected valuesof the peak current Ip and the optimum cutting condition are prestoredin an electronic computer for each workpiece thickness. The thickness ofa workpiece to be cut is estimated from an actual cutting speed andcutting condition through the use of the computer and compared with theprestored optimum cutting condition and then the peak current Ip ischanged over to an optimum value. However, this prior art equipmentrequires, in addition to the use of an electronic computer, a largenumber of experiments for obtaining the data to be prestored in thecomputer; furthermore, since all the data thus obtained must beprestored, their processing is very troublesome.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an automatic cuttingcondition changeover equipment which does not require an electroniccomputer and is easy to handle.

Another object of the present invention is to provide an automaticcutting condition changeover equipment which is designed to lighten thecutting condition when cutting is unstable so as to prevent the breakageof the wire electrode.

Briefly stated, the automatic cutting condition changeover equipment ofthe present invention is provided with a feed control circuit whichcontrols the cutting speed so that the mean working voltage between thewire electrode and the workpiece is constant, an arithmetic logic unitwhich is supplied with the cutting speed, the peak current or the meanworking current of the power source of a wire-cut, electric dischargemachine and constants determined by cutting conditions other than thepeak current or mean working current and which performs calculationsaccording to a predetermined optimum cutting condition formula, andelectrical cutting condition changeover means which changes the peakcurrent so that the output from the arithmetic logic unit is reduced tosubstantially zero. In the case where the peak current remains unchangedalthough the thickness of a workpiece has increased, the cutting speedlowers and, consequently the result of the calculation of the optimumcutting condition formula becomes minus and thus the peak current isincreased by the electrical cutting condition changeover means.Conversely, when the peak current remains unchanged regardless of adecrease in the thickness of the workpiece, since the cutting speedincreases, the calculation result of the abovesaid formula becomes plusand thus the peak current is decreased by the electrical cuttingcondition changeover means.

Further, a detector is provided for detecting variations in the meanworking voltage, and the constants of the optimum cutting conditionformula are altered in accordance with the variations in the meanworking voltage to prevent the breakage of the wire electrode at a placewhere cutting is unstable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electric circuit diagram showing a general arrangement of apower source for use in wire-cut, electric discharge machines; and

FIG. 2 is a block diagram illustrating the principal part of anembodiment of the automatic cutting condition change-over equipment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is depicted an electric circuit diagram of a powersource of a wire-cut, electric discharge machine employed for obtainingan optimum cutting condition formula which is used in the presentinvention. Reference character Vs indicates a no-load voltage; Rsdesignates a charging resistor; Ip identifies a peak current; Q denotesa transistor; P represents a pulse signal for driving the transistor Q;τ_(on) and τ_(off) show the ON time and the OFF time of the pulse signalP; C refers to a capacitor; WR indicates a wire electrode; and WKdesignates a workpiece. According to the present inventor's experiments,a maximum cutting speed Fm and a peak current Ipm that would not causeshorting between the wire electrode WR and the workpiece WK or breakageof the wire electrode WR were obtained for workpieces of differentthicknesses but of the same material. These results were obtained usingthe power source circuit of FIG. 1 in which the wire electrodes usedwere made of the same material and had the same diameter and the no-loadvoltage Vs, the ON time τ_(on) and the OFF time τ_(off) of the pulsesignal P and the capacitance of the capacitor C were held unchanged. Ithas been found that the following expressions hold over a wide range ofcutting speeds: ##EQU1## In the above expressions, h is the thickness ofthe workpiece and K₁, K₂ and F₀ are constants which depend on electricalcutting conditions other than the peak current Ip, such as the materialsof the workpiece and the wire electrode, and the diameter of the wireelectrode. In the expressions (1) and (2), even if the cuttingconditions other than the peak current are modified, only the constantsK₁ and K₂ undergo change and the forms of the expressions do not change.

Eliminating the thickness h of the workpiece from the expressions (1)and (2), the following expression is obtained:

    (Fm+F.sub.0)Ipm=K                                          (3)

where K=K₁ K₂ and the constant K is one that becomes large or smalldepending on whether or not the material of the workpiece is easy tocut. The constant F₀ varies with the value of the constant K accordingto experiments but it can be estimated approximately by the followingexpression:

    F.sub.0 =F.sub.1 +αK                                 (4)

where F₁ is a constant which is determined by the electric circuitarrangement used and α is a sufficiently small constant. Accordingly,the constant F₀ can be regarded as constant in practice and it is fixedfor each machine.

The present invention is intended to automatically change over theelectrical cutting condition in accordance with the thickness of aworkpiece on the basis of the following principles, using the abovesaidexpressions (3) and (4).

At first, the feed of the wire electrode and the workpiece relative toeach other is controlled so that the mean working voltage between themis constant. Such feed control has heretofore been carried out forpreventing the width of the groove of a cutting path varying with thethickness of the workpiece so as to degrade the cutting accuracy, andmany methods are known; accordingly, this control is effected by any ofthe known methods. During the feed control for making the mean workingvoltage constant, if a peak current Ip which is smaller than the Ipm ofthe maximum cutting speed condition given by the expression (3) isprovided, (that is, if the peak current Ip remains unchanged regardlessof an increase in the thickness of the workpiece) then the cutting speedF decreases and (F+F₀)Ip becomes smaller than the constant K in themaximum cutting speed of the expression (3). Conversely, when the peakcurrent Ip is too large, the cutting speed F increases, resulting in(F+F₀)Ip>K. Accordingly, by controlling the feed to make the meanworking voltage constant and, at the same time, detecting the cuttingspeed F and the peak current Ip and checking whether they satisfy theexpression (3), the thickness of the workpiece can be determined. Thusby increasing or decreasing the peak current Ip to meet the expression(3), the electrical cutting condition is automatically changed over inaccordance with the thickness of the workpiece.

FIG. 2 illustrates, in block form, the principal part of an embodimentof the present invention. Reference characters CNT1 and CNT2 indicatecounters; REG1 to REG3 designate registers; ART identifies an arithmeticlogic unit; CMP1 and CMP2 denote decision circuits; DET represents adetector; G, G1 and G2 show AND circuits; Ip refers to peak current; fpindicates feed pulses; cp designates a feed rate command value; spidentifies sampling pulses; and gp denotes gate pulses.

In FIG. 2, the present cutting speed F is detected by counter CNT1counting the feed pulses fp corresponding to a unit feed of the worktable or by setting the feed rate command value cp from a numericalcontrol unit in the counter CNT1 and then the cutting speed F thusdetected is provided to the arithmetic logic unit ART. Likewise, thepresent peak current Ip is set in the register REG1 and then applied tothe arithmetic logic unit ART. The register REG2 has set therein thevalue K obtained experimentally and the register REG3 has set thereinthe value F₀ which is determined by the discharge circuit employed inthe power source of the wire-cut, electric discharge machine, and thesevalues are also applied to the arithmetic logic unit ART.

The arithmetic logic unit ART calculates (F-F₀)Ip-K from the cuttingspeed F, the peak current Ip and the values K and F₀, and provides thecalculation result to the decision circuits CMP1 and CMP2. The decisioncircuit CMP1 decides whether the calculation result is plus or not and,in the case of plus, decreases the value of the counter CNT2. Thedecision circuit CMP2 decides whether the calculation result is smallerthan -δ or not and, if smaller, increases the value of the counter CNT2.δ is used to provide a hysteresis operation so that the value of thecounter CNT2 may not frequently vary and it is set to a valuesufficiently smaller than the value K.

The output from each stage of the counter CNT2 is provided via the ANDcircuit G to peak current changeover means (not shown) to obtain thepeak current value proportional to the content of the counter CNT2. Asthe peak current changeover means, use can be made of, for example, anarrangement in which several transistors Q such as shown in FIG. 1 areconnected in parallel and the number of their operations is changed bythe output from the counter CNT2, or an arrangement in which the valueof the charging resistor Rs is changed by the output from the counterCNT2.

With the above-described arrangement, the peak current Ip canautomatically be changed over in accordance with the thickness of aworkpiece. The circuit shown in FIG. 2 is constructed so that avariation in the mean working voltage Vw is detected by the detector DETand the value K is reduced in accordance with the variation. Thedetector DET compares the variation between the mean working voltage Vwand the voltage Vw-STANDARD LEVEL, and if the variation is too high,provides a reducing signal to REG 2. The reason is as follows: In awire-cut, electric discharge machining, cutting becomes unstable at acorner or a small-radius, and the wire electrode is liable to be broken.Accordingly the peak current Ip and other cutting conditions must belightened at such portions of the workpiece, and since the mean workingvoltage usually undergoes a substantial change at the portion where thecutting is unstable, the value K is decreased by using the change in themean working voltage. This permits stable cutting of the workpiece evenat corners.

In the above embodiment the peak current Ip is measured but since it hasbeen found by experiments that a relation similar to the expression (2)also holds for a mean working current Iw, it is also possible to measurethe mean working current Iw instead of the peak current Ip in FIG. 2 andautomatically change over the peak current Ip in accordance with thethickness of the workpiece so that (F-F₀)Iw-K=0.

As has been described in the foregoing, since the equipment of thepresent invention is constructed based on the experimental formula forthe optimum cutting condition, no electronic computer is needed.Moreover since the value F₀ is fixed for each machine, it is sufficientonly to input thereto the value K obtainable by simple experiments;therefore, the equipment of the present invention is far easier tooperate than in the prior art. In general, the value K is experimentallyobtained for each of various cutting conditions on the part of themachine maker and it is sufficient for the user to set the value K basedon the experimental values. Even in the case where the cuttingconditions such as the material of the workpiece, the capacitance of thecapacitor and so forth have been changed, a new value K is determinedonly by experimental cutting of one workpiece and the peak current canautomatically be changed over using the new value K. Accordingly, evenin the case where the workpiece is made of a special material and nodata on the value K are available from the machine maker, the workpiececan be cut with ease.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts of thisinvention.

What is claimed is:
 1. An automatic cutting condition change-overequipment having a power source and a wire electrode, and having a meanworking voltage, a cutting speed, and a cutting speed signal responsiveto the cutting speed for controlling the cutting speed of a workpiece,comprising:a feed control circuit, operatively connected to receive thecutting speed signal, to the workpiece and to the wire electrode, forcontrolling the cutting speed so that the mean working voltage betweenthe wire electrode and the workpiece is constant; an arithmetic logicunit, operatively connected to receive the speed signal and a peaksignal indicating the peak current of the power source, for calculatinga predetermined optimum cutting condition formula in accordance with thepeak signal and the speed signal, and for providing, as an output, theresult thereof; and electrical cutting condition changeover means forchanging the peak current of the power source so that the output fromthe arithmetic logic unit is reduced to substantially zero.
 2. Anautomatic cutting condition changeover equipment according to claim 1,wherein the optimum cutting condition formula is (F+F₀)Ip=K, where F isthe cutting speed, Ip is the peak current and F₀ and K are predeterminedconstants.
 3. An automatic cutting condition changeover equipmentaccording to claim 2, wherein the electrical cutting conditionchangeover means comprises:a decision circuit operatively connected tothe arithmetic logic unit, for deciding the value of the optimum cuttingcondition formula; a counter operatively connected to the decisioncircuit, for counting up or down in accordance with the output from thedecision circuit, and; peak current changeover means operativelyconnected to the counter, for changing the peak current in accordancewith the output from the counter.
 4. An automatic cutting conditionchangeover equipment according to claim 3, which further comprises:adetector operatively connected to the arithmetic logic unit, fordetecting a variation in the mean working voltage, and wherein the valueK in the optimum cutting condition formula is decreased in accordancewith the variation in the mean working voltage.
 5. An automatic cuttingcondition changeover equipment according to claim 1, wherein the optimumcutting condition formula is (F+F₀)Iw=K, where F is the cutting speed,Iw is a mean working current and F₀ and K are predetermined constants.6. An automatic cutting condition changeover equipment according toclaim 3, wherein the decision circuit further comprises: hysteresismeans operatively connected to the arithmetic logic unit, for increasingthe counter in accordance with the output of the arithmetic logic unit.